A common method for detecting currents flowing in three phases of a three-phase inverter which serves as a power converting apparatus has been to detect currents of two phases by use of current sensors provided in two of the three phases and calculate a current of the remaining one phase utilizing the fact that the sum of the three-phase currents is zero, thereby detecting all of the three-phase currents.
There however arises a problem that the provision of the two current sensors results in a corresponding increase in cost as well as an increase in volume. A method developed to solve this problem is to provide one current sensor in a DC bus line of an inverter and reproduce three-phase currents from a DC bus current and switching states of semiconductor switching devices of the inverter.
Specifically, if currents are detected in a switching pattern of the DC bus current with which two different phase currents flow, it is possible to detect the three-phase currents utilizing the fact that the sum of the three-phase currents is zero (refer to Patent Document 1, for example).
The aforementioned method however has had a problem that if three-phase voltage commands are small or a difference between voltage commands for two phases among the three-phase voltage commands are small, the duration of time of the switching pattern becomes shorter, making it difficult to detect the currents.
A method developed under such circumstances is to make such a correction that the difference among the three-phase voltage commands becomes large enough to detect the currents in a first half of each pulse width modulation (PWM) cycle as well as such a correction in a second half of each PWM cycle that an average of voltage commands in the first and second halves becomes equal to an original voltage command. According to this method, it is possible to calculate the three-phase currents by detecting currents of two different phases in each PWM cycle (refer to Patent Document 2, for example).